The present invention relates to a hands-free telephone and, more particularly, to a hands-free telephone advantageously applicable to a automobile telephone or similar mobile telephone system.
It is a common practice with an automobile or car telephone or similar mobile telecommunications equipment to allow conversation to be selectively held on either a mouthpiece and an earpiece built in a handset or an optional hands-free telephone.
A hands-free telephone for on-board use includes an antenna and a radio section which are incorporated in a handset, and a hands-free telephone unit fixed in place within the vehicle cabin. The antenna transmits and receives a radio signal from a base station. The radio section includes a transmitter for transmitting a transmit signal by converting it into a radio signal, a receiver for converting a received radio signal into a received signal, and an antenna duplexer for coupling or uncoupling the transmit and receive radio signals. A variable resistor is provided for adjusting the level of a receive signal to be fed from the receiver to the earpiece. The hands-free telephone unit has a microphone for converting transmit voice into a transmit signal, a first variable attenuator, or variolosser, for attenuating the level of the transmit signal from the microphone in response to a control signal, a second variable attenuator, or variolosser, for attenuating the level of the receive signal from the receiver, a comparator for producing a control signal in response to the transmit and receive signals from the microphone and second variolosser, respectively, and a speaker for converting the receiver signal into voice. While the hands-free telephone is not used, a transmit signal input terminal and a receive signal output terminal of the radio section are connected to, respectively, a transmit and a receive circuit of the handset.
Generally, a hands-free telephone suffers from howling or singing ascribable to the acoustic coupling between a microphone and a speaker. Usually, therefore, a hands-free telephone is constructed such that when transmit voice is inputted, the attenuation by the second variable attenuator is increased while, when receive voice is outputted, the attenuation by the first variable attenuator is increased. The loop gain of the telephone communications loop including the acoustic coupling circuit between the microphone and the speaker is decreased to eliminate howling or singing.
The hands-free telephone unit usually has a variable resistor for adjusting the volume of voice coming out of the speaker, and such a variable resistor of course needs an extra space for installation. When the hands-free telephone is used as an on-board telephone and the hands-free telephone unit thereof is installed in the vehicle cabin, it is necessary that the above-mentioned variable resistor be located within the user's reach. However, locating the variable resistor having a substantial size in the limited space available in the vehicle cabin is objectionable from the comfortableness and appearance viewpoint.
In the light of the above, the receive signal whose level has been changed by the variable resistor built in the handset may be used as a receive signal to be applied to the hands-free telephone unit. Such a scheme, however, would also change the level of the received signal to be fed to the comparator and would thereby render the volume control at the time of transmission/reception switching unstable.